The present invention relates to mask lenses for night vision and optical sighting. More particularly, the present invention relates to a device and method for a non-uniform lens which incorporates geometry for external and internal eye relief compatible with optical sighting and optical correction systems.
This invention relates to a method and device for a single non-uniform mask lens which incorporates eye relief geometry for night vision and optical sighting systems as well as for optical correction systems.
In existing military masks, such as the M40 and MCU-2/P, eye relief of the mask lens is well outside the 25-mm eye relief needed for night vision goggles and a variety of sighting systems. In military masks such as the M48/M49, the mask lenses are close enough to the eye to provide eye relief but do not adequately accommodate optical correction. The existing M45 mask provides adequate internal and external eye relief but requires a two-lens system.
Available optical geometry dictates the use of common geometrical shapes in the development of lens systems. These common geometrical shapes do not allow for optimal internal and external eye relief needed for compatibility of optical correction with optical sighting systems. A similar lens profile to that of the present invention could be developed by combining common optical geometrical shapes. However, available optical polishing techniques do not allow for a smooth optical transition between lens geometries.
The present invention provides a device and method which incorporate an optimal interface between the internal optical correction and external sighting systems in a single non-uniform mask lens. The non-uniform mask lens can be made of any optical material and can be cast or injection molded to incorporate the desired geometry.
An embodiment of the present invention employs the polynomial curve used to describe the Phase III RESPO 21 lens, a lens design that has been established as the leading lens design approach for certain military applications, see Edgewood Research, Development, and Engineering Center Technical Report 368 (ERDC-TR-368), incorporated herein by reference in its entirety and U. S. patent. application Ser. No. 9/049,659, xe2x80x9cAdvanced Chemical Biological Mask,xe2x80x9d also incorporated herein by reference in its entirety. In this design approach, a polynomial curve captures the geometry of the specific mask lens design illustrated in FIG. 1. Table 1 contains the coordinates for the contour 10 of this particular lens generated using the polynomial curve established for this lens design. Using this polynomial curve, points generated to define the outside profile 20 of the overall geometry of this lens are contained in Table 2. A first best-fit polynomial curve for the front surface is generated from these coordinates and is then combined with optical calculations to generate a second polynomial curve for the back surface. The combination of these two curves adds the optical geometry required for eye relief to the original mask lens, see C. M. Grove, et al, Edgewood Chemical Biological Center Technical Report 063 (ECBC-TR-063), xe2x80x9cLens Concept For The Joint Service General Purpose Mask,xe2x80x9d October 1999, incorporated herein by reference in its entirety.
The resultant family of curves is
Sag(x)=ax2+bx4+cx6+dx8+ex10+fx12
where the coefficients (a, . . . , f) of the various powers of x are contained in Table 3. The resultant curves for the front and back surfaces of the lens fully define the geometry needed for the variable thickness lens that is optically corrected to accommodate changes in curvature. That is, both curves fully define the geometry needed for the variable thickness lens with this contour geometry. Using this equation and the coefficients contained in Table 3, the thickness corresponding to specific points on the lens can be calculated, as contained in Table 4.
Alternatively, an embodiment of a non-uniform lens according to the present invention having both a front surface curvature and a back surface curvature can be approximated using a double conic geometry. However, this double conic geometry requires optical blending in the middle of the lens. Required blending can be minimized by forcing tangency and curvature at the mating surfaces in this embodiment.
The non-uniform lens can be made of any optical material. A flat variable thickness mold can be employed, or, alternatively, the lens can be fabricated using injection molding.
The device and method according to the present invention provide a variable thickness mask lens with a contoured optical surface which provides both eye relief for night vision and optical sighting and optical correction to compensate for changes in lens curvature.